1. Technical Field
The technical field relates to a zoom lens and an imaging apparatus suitable for contrast AF (Autofocus).
2. Related Art
In recent years, a digital single-lens reflex camera capable of converting an optical image of a subject into an electric image signal so as to record it to a recording medium such as a semiconductor memory. With such a digital single-lens reflex camera, a user can take a picture of a subject while viewing the subject through either of an optical finder and a liquid crystal display monitor provided to a rear side of the camera. When the user views the subject through the finder, light (namely, a subject image) incident on a lens is reflected by a reflection mirror arranged on an optical path after the lens so that the optical path is changed. Then the subject image is changed into an erected image through a pentaprism and is led to the optical finder, so that the user can view the subject image passing through the lens from the optical finder.
As the spread of digital single-lens reflex cameras among users, a small, light-weighted and inexpensive camera such as a compact camera is more demanded. However an optical system, including a reflection mirror and a view finder cause a problem in view of downsizing. Therefore, recently the digital single-lens camera appears, which is not provided with a reflection mirror to allow a subject image always to be imaged on an imaging sensor so that the user can take a picture while viewing the subject image through a liquid crystal display monitor or an electronic viewfinder provided on the rear side of the camera.
When a group of four lenses with high magnification are used as the zoom lens, speed and accuracy of focusing at the time of the zooming become a problem. The above digital single-lens camera employs contrast type autofocus (“contrast AF”) which always uses an image signal from an imaging sensor which is being captured in real time, thus providing more accurately focusing than employing phase difference type autofocus.
Conventionally, the following mechanism is proposed (for example, see JP07-333482A). In this mechanism, a feed mechanism for feeding a focus lens in the group of four zoom lenses of an inner focus type in order to provide zoom tracking function mechanically, and a feed mechanism for feeding the focus lens for focusing are shared to be downsized, so that a focus operation can be performed smoothly on an entire zooming area. Further, another method is proposed (for example, see JP2-266312A), which calculates a movement of a focusing system with high accuracy and at high speed in a proper calculation method to realize electrically zoom tracking of the group of four zoom lenses.
FIGS. 13 to 15 are diagrams describing a configuration of a conventional zoom lens disclosed in JP7-333482A. A conventional zoom lens 100 shown in FIG. 13 includes a first lens-group 101 with positive refractive power, a second lens-group 102 with negative refractive power, a third lens group 103 with positive refractive power, and a fourth lens group 104 with positive or negative refractive power. As shown in FIG. 13, a zoom ring 110, a focus ring 120, a first lens-group moving frame 130, a first variable cam barrel 140, a direct advancing barrel 150, a first lens-group moving barrel 60, a fixed barrel 70 integral with a lens mount 105, a second variable cam barrel 80 and a second lens-group moving frame 90 are provided to a lens barrel 106 to be mounted to a camera body (not shown) with the lens mount 105, in this order generally from the outside.
The zoom ring 110 is connected to the second variable cam barrel 80 with a first connecting pin 111P, and rotates integrally with the second variable cam barrel 80. A focus operation sliding pin 121P is mounted to the focus ring 120 on a side of the lens mount 105, and rotates according to the rotation of the focus ring 120.
The first lens-group moving barrel 60 is provided with a third cam groove (focus cam) 62 of the second lens-group engaged with a second lens-group guide pin 91P, and a focus operation sliding pin guide groove 63 engaged with a focus operation sliding pin 121P. FIG. 14 illustrates a relationship among the third cam groove 62 of the second lens-group, the second lens-group guide pin 91P, and the focus operation sliding pin guide groove 63. The second variable cam barrel 80 is provided with a second lens-group guide groove 85 that is engaged with a second lens-group rotating pin 92P and transmits a rotational movement to the second lens-group rotating pin 92P (see FIG. 15).
Returning to FIG. 13, the second lens-group moving frame 90 is provided with the second lens-group guide pin 91P on the subject side and with the second lens-group rotating pin 92P to be engaged with the second lens-group guide groove 85 on the lens mount 105 side. The second lens-group guide pin 91P is engaged with the third cam groove 62 of the second lens-group. FIG. 15 illustrates a relationship among the second variable cam barrel 80, the second lens-group rotating pin 92P, the second lens-group guide groove 85, and the second lens-group guide pin 91P.
An operation of the zoom lens 100 will be described with reference to FIGS. 13 to 15.
When the zoom ring 110 rotates, the second variable cam barrel 80 rotates via the first connecting pin 111P. Due to the rotation of the second variable cam barrel 80, the third lens-group 103 is moved by the third lens group guide pin 103P engaged with a cam groove provided to the second variable cam barrel 80 and a cam groove provided to the fixed barrel 70.
The rotation of the second variable cam barrel 80 rotates and moves forward and backward the second lens-group moving frame 90 and the second lens-group guide pin 91P according to control of the second lens-group rotating pin 92P engaged with the second lens-group guide groove 85 and the second lens-group guide pin 91P engaged with a second cam groove 52 of the second lens-group provided to the straight advancing barrel 150. When rotating, the second lens-group guide pin 91P moves along the third cam groove 62 of the second lens-group. The other lens groups 101 and 104 similarly move in a predetermined manner according to the rotation of the zoom ring 110, so that the zoom operation according to a rotational angle of the zoom ring 110 is realized.
On the other hand, when the focus ring 120 rotates, this rotation is transmitted to the first lens-group moving barrel 60 via the focus operation sliding pin 121P and rotates the first lens-group moving barrel 60. The rotation of the first lens-group moving barrel 60 moves the first lens-group moving barrel 60 forward and backward due to the engagement between the second lens-group guide pin 91P and the third cam groove 62 of the second lens-group. As a result, the second lens-group 102 moves forward and backward.
In order to focus on an infinite (∞) subject in a hill-climbing contrast AF method, a down-hill area for detecting a peak, namely, an area for moving the focus lens group beyond infinity (∞) is necessary.
An AF evaluation value used for the contrast AF abruptly changes according the position of the focus lens when a zoom position is on a telephoto side. However, when the zoom position is on a wide-angle side, the AF evaluation value gently changes. For this reason, particularly when the zoom position is at a wide-angle end, the focus lens group should be moved in a wide range in order to detect a peak accurately.
A movable range of the focus lens group is restricted to a length of the third cam groove 62 of the second lens-group. Specifically, when the zoom position is at the wide-angle end and the subject distance is infinite (∞), an area close to an end of the third cam groove 62 of the second lens-group is used. In this state, the movable range of the first lens-group moving barrel 60 rotating in conjunction with the focus ring 120 is restricted, and the movable range of the focus lens group is restricted. For this reason, the focus lens group cannot be moved along a sufficiently long distance for down hill. That is, in the contrast AF, the focus lens cannot be moved in a distance necessary for obtaining the peak of the AF evaluation value. In other words, with conventional zoom lenses, the AF evaluation value cannot be accurately obtained, when the contrast AF is performed with the zoom position at the wide-angle end and the infinite (∞) subject distance.